I got a software to edit subtitles - here
I took the first one (Aegisub) and installed it.
I wanted to shift the time of subtitles forward by 20 Seconds. It is pretty simple.
Just open the file with Aegisub and click on "Open Shift Times Dialogue" - the icon on toolbar with the pic of an arrow and a vertical stripe. Enter the required values.
Now I can watch any movie with better understanding. \0/
By the way, if you have a movie without subtitles, then search for subtitle file of that movie on internet. Some of them have .srt extension. (I used the same.) There can be .sup too. I don't know what the difference is. But .srt worked for me.
Take the file. Rename it with the same name as that of your movie file name. (keep the extension as .srt; don't change that).
Keep both the movie file and srt file in the same folder.
Open your movie player. I used VLC player. In that, select Video menu-> Subtitles Track -> Track 1.
It should work. (You might need to restart the VLC player.)